1. Field of the Invention
This invention relates to a small-sized fluid treatment apparatus for filtration and separation of fluid, transferring substances between fluids through membranes or passing specific substance contained in the fluid through membranes. Said apparatus consists of a plurality of stacked membrane arrangements which comprise membrane support-plates, each being molded in thickness of 1.2 mm or less by means of embossing or press work, and a single membrane interposed between said membrane support-plates. The membrane support-plate composing the membrane arrangement of this apparatus is designed such that the 1st fluid is fed into the space defined by the 1st membrane support-plate and the membrane, and the 2nd fluid is fed into the space defined by the 2nd membrane support-plate and foresaid membrane, with no abruptly narrowed passage for fluid.
2. Description of Prior Art
Fluid treatment apparatus employing membranes are in wide use for various purposes. For example, for industrial use there are the condensing, purifying apparatus, ion exchangers, etc., artificial kidneys, artificial lungs, ascitic treatment apparatus, etc. for medical use, and purification of test pieces, bio-reactors, etc. for analytical use. Particularly, the artificial kidney has been the one attracting public attention in recent years.
Among various ways of use of a membrane, a sheet membrane is used for a comparatively large surface area of a unit construction such as sheet film in the case of the so-called Kiil dialyzer. The apparatus according to this invention is then designed to be adapted preferably for the artificial kidney. Accordingly, the invention will be described hereunder by way of examples of artificial kidneys. However, it will become apparent that the application of the present invention extends to other apparatus for filtration and separation of fluid.
The Kiil dialyzer is an apparatus used for blood purification where dialysate and blood are introduced into channels formed by a plurality of flat membranes and a plurality of stacked membrane support-plates, transferring effete matters, poisonous matters, and water from blood to dialysate through the membranes utilizing diffusion resulting from a concentration gradient and ultrafiltration from the pressure gradient between the blood and dialysate.
As compared with the coil and the hollow fiber dialyzer, the Kiil dialyzer is appreciated and used as a safe and excellent apparatus having the following advantages:
(1) Due to a small resistance to blood flow, little damage is caused to blood cells.
(2) Blood loss is small as remaining blood after use is small.
(3) Because the priming volume of the Kiil dialyzer is essentially small, blood taken out of the body of a patient is small, thus causing no disorders in a patient's body such as anemia.
A typically known conventional Kiil dialyzer such as disclosed by U.S. Pat. No. 3,837,496 comprises a plurality of stacked membrane support-plates, a plurality of paired membranes interposed between said membrane support-plates, and blood ports having radial blood distributing grooves between paired membranes. Dialysate flows in the space formed by the membrane support-plate and the membrane, while blood introduced through blood ports flows between a pair of membranes. In another type of Kiil dialyzer as disclosed by U.S. Pat. No. 3,585,131, the membrane support-plates are inserted into the pleat on one side of the folded membrane to make up a passage of dialysate, while the other support-plates alternatively form a blood passage into the pleat on the other side. However, in most of these known conventional artificial kidneys, the membrane support-plates are usually manufactured by sophisticated injection molding in order to achieve the smooth but complicated blood passage which is necessary to avoid damage to blood. The thickness of the membrane support-plate thus obtained naturally falls in the range of from 1.2 mm to 5.0 mm when a complicated configuration of fluid passage, and economized speed for injection molding, are taken into consideration. As a result, the Kiil dialyzer which employs layers of such injection molded support-plates has such drawbacks as being bulky, heavy and difficult in handling in the hospital. In addition, a large amount of used resin, which is the material of the support-plates, may cause disposal problems after their use, as in the case of incineration. On the other hand, construction of the former artificial kidney requires exact positioning of a blood port between a pair of membranes and of membranes on the support-plate for stacking. Such an operation is extremely difficult to automate, which means inadaptability to mass production system. As regards the artificial kidney described secondly, it may permit automated stacking of the membranes and the support-plates because of no need of providing blood ports but it requires support-plates instead in order to form a blood passage. It therefore has disadvantages of being bulky, almost twice as large as that of the one described first and of a large amount of resin consumption. So, the known conventional Kiil dialyzers may tend to be bulkier and more expensive than the coil and hollow fiber dialyzers. However, since the Kiil dialyzer is safe and exhibits excellent performance, a new type of Kiil dialyzer is desired which is comparable to and even superior to coil and hollow fiber dialyzer in compactness, weight and cost.
The present inventors thoroughly examined the mechanism, design, construction and assembly of the known Kiil dialyzer in an attempt to achieve the foresaid requirements. We have found:
(1) that in order to exhibit the excellent performance of a Kiil dialyzer, dialysate and blood must flow through channels properly formed on the support-plate without uneven flow, chanelling and/or by-passing, etc. For this purpose, support-plates must not be eliminated.
(2) that, although mesh is used as a membrane-support for a coil dialyzer, it is not recommended in case of Kill dialzyer. Our investigation reveals that the mesh easily captures bubbles, which leads to blood coagulation and loss as well as causing uneven flow, probably due to the woven pattern.
(3) that in the Kiil dialyzer having blood ports, positioning blood ports between a pair of membranes may prevent automated process of manufacture and therefore is a limit in making it compact, light and inexpensive. It is, then, necessary to eliminate blood ports between a pair of membranes. Support-plates instead should be employed so as not only to give blood passage but also to enable stacking process automated for mass production.
(4) that in order to employ a support-plate instead of a blood port and furthermore to make an assembly compact and inexpensive at the same time, it is important to reduce the thickness of each support-plate. Reduction of the apparatus in size and of the cost, then, depends upon the thickness of support-plate. We need to seek a better support molding method to achieve this because, with the conventional injection molding, it is difficult to obtain a support-plate less than 1.2 mm in thickness with economized speed for mass production.
(5) that molding of a support-plate by embossing or press work on a thermoplastic film may permit the thickness to be reduced to less than 1.2 mm and at the same time a lower cost.
However, a conventional injection molding technique is not suited for molding support-plates of thickness less than 1.2 mm in an economical production speed because of the difficulty in molding the complicated fluid channels. Therefore, in order to fulfil the requirements, it is necessary to adapt another molding method. By our further study, we have found that embossing and press work may permit accurate molding of thinner support-plates. Moreover, embossing a support-pattern on a relatively inexpensive thermoplastic film can be made at fairly high speed, which enables the cost reduction of a support-plate and consequently the cost reduction of a dialyzer. With this embossing technique, we attempted a manufacture of the support-plate having the same design as disclosed in U.S. Pat. No. 3,585,131, but of 1.0 mm in thickness. We observed that the support-plate thus obtained caused uniform flow of blood and dialysate and abnormal pressure increase, which resulted in poor performance of the dialyzer. The bubbles captured in the fluid passage are considered to be responsible for the poor performance. It is, then, found virtually unpractical to reduce the plate thickness simply without a change in the design of the fluid channels and the membrane supporting pattern as employed in the U.S. Pat. No. 3,585,131. From our experimental study, we have found that there were less bubbles trapped in the fluid passage for a support-plate thicker than 1.2 mm. As it becomes thin, e.g. less than 1.2 mm, bubble trapping started and then dialyzing performance began to decrease. Remarkable decrease in the performance was seen when the thickness was less than 0.8 mm.
Trapping of bubbles in the fluid channels resulting from the reduction of the plate-thickness may be construed as follows:
The bubble resists deformation from its spherical shape by the surface tension of the ambient fluid. The smaller the bubble is, and the larger the degree of deformation is, the larger the resistance becomes. Therefore, the thinner the support-plate is, the smaller the bubble becomes and the larger the resistance to deformation becomes. In the membrane support-plate of a conventional Kiil dialyzer construction, a bubble can not be removed unless it is divided into two or more parts, or even deformed so as to be halved at least in the direction of the thickness of the support-plate. In the case of such abrupt, so to speak, intermittent deformation a bubble, however, the smaller the bubble is, the greater the resistance thereof becomes. Accordingly, when there is a portion where the fluid passage is abruptly narrowed as in the simple reduction of the thickness of support-plate of the conventional Kiil dialyzer, it is extremely difficult to discharge bubbles smoothly, thereby allowing for retaining of bubbles in the course of the fluid flow.
Based upon the above consideration and experimental results, we have achieved the design of a dialyzer according to the present invention. The detailed construction of its constituting members will be described hereinafter.